In the initial stages of conventional silicon integrated circuit processing, a silicon substrate is covered by a pad oxide and a silicon nitride layer. The silicon nitride and pad oxide are subsequently patterned to exposed predetermined regions of more or less bare silicon. An oxidation step is performed to create a field oxide, sometimes called an isolation oxide, in the regions where bare silicon had been exposed. The above-described process is often termed the LOCOS process ("local oxidation of silicon").
A variant of the LOCOS process is the poly-buffered LOCOS process in which a layer of polysilicon is formed between the pad oxide and the silicon nitride layer. The polysilicon layer helps to relieve stresses created during the growth of the field oxide.
In both the LOCOS and the poly-buffered LOCOS processes, after the field oxide is grown, it is desirable to remove both the silicon nitride and the polysilicon layer. (Frequently, the underlying pad oxide is also removed.) Typically, the silicon nitride layer is removed by etching in hot phosphoric acid. The polysilicon layer is subsequently removed by a dry etch process. Unfortunately, the phosphoric acid wet etch often used to remove the silicon nitride creates small pits in the polysilicon. The dry etch process which is subsequently utilized to remove the polysilicon, being anisotropic, transfers the pits in the polysilicon downward into the pad oxide, or, under over-etch conditions, into the silicon substrate. This pitting of the pad oxide and the underlying substrate is undesirable.